The anomalous magnetic moment of the muon

a  = g-22

provides a precision test at low energies.
The experimental result of E 821 at Brookhaven National
Laboratory [1]} has reached a substantial improvement in accuracy.
It shows a deviation from the Standard Model prediction by 3–4
standard deviations depending on the evaluation of the hadronic
vacuum polarization from data based on \(e^+e^-\)  annihilation
as shown in Fig. REF  [2]}.
For a recent review see Ref. [3]}.
